The remarkable ability of the human auditory system to extract meaning from continuous human speech can be more fully understood in relation to listeners' abilities to "hear out" the details of complex sounds in general. The proposal research will determine the limits of auditory discrimination and identification abilities of normal listeners for a variety of complex sounds. These experiments first extend earlier work with tonal patterns and develop a more recent line of investigation with spectrally complex sounds. The proposed studies with tonal patterns are divided into three categories, each addressing a basic issue in the discrimination or identification of complex sounds. These are 1) the ability to discriminate between complex patterns with various types of spectral and temporal microstructure; 2) the ability to recognize relational properties that form the invariant that define a pattern under various types of transformations; and 3) the ability to resolve pattern detail under different levels of pattern uncertainty (or familiarity). A second series of experiments will investigate the generality of the proportion-of-the-total-duration (PTD) rule, which has been shown to be a remarkably strong predictor of the deductibility of changes in components of a variety of unfamiliar complex sounds. Two recent theoretical model for the discrimination of complex sounds, one based on informational content and the other on the physical properties of the stimulus are able t o predict the general form of the PTD-rule results. Experiments will be conducted to determine which of these models provides the more comprehensive account of the ability to resolve the details of complex sounds. A third series of experiments will determine the abilities of listeners to resolve the details of dynamically varying, spectrally complex sounds, using non-speech stimuli that include some of the properties of speech. These studies will investigate the effects of various types of spectral complexity, including a new class of experiments on listeners' abilities to recognize the characteristics of complex filters. An improved version of the Test of Basic Auditory Capabilities will also be developed.